This invention relates to tracking systems for aiming a laser beam and/or positioning projected light patterns at a known relation onto moving targets. In particular, the invention is concerned with the detection of, measuring of and compensating transverse movements of optical targets such as an eye during ophthalmic laser surgery as well as non-surgical diagnostic procedures. The present invention is particularly powerful when taken in conjunction with a method for capturing, measuring and compensating for movements along the axial direction such as disclosed by Wm. D. Fountain in U.S. Pat. No. 5,162,642 which is assigned to the same party as the present invention.
Methods of integration of a lateral tracker with depth ranging techniques were disclosed in copending patent application Ser. No. 843,374, entitled xe2x80x9cAutomated Laser Workstation for High. Precision Surgical and Industrial Interventionsxe2x80x9d, which was filed on Feb. 27, 1992, and which is incorporated herein by reference. In that disclosure, fully automated means to acquire and track randomly moving targets in three dimensions were described, along with methods for interfacing the acquisition and tracking means with a beam aiming and targeting sub-system and a target viewing subsystem, all of which are elements of a complete laser workstation. It is noteworthy that the system and method of said patent application also included, in an alternate embodiment, the capability to distinguish between translational and rotational movements of the eye as an integral part of the full three dimensional tracker.
By comparison with the two earlier disclosures cited above, the present invention emphasizes those aspects and specific embodiments of a transverse 2D tracker that are most critical in allowing a laser beam or projected light patterns to be correlated with and/or directed to a specific location on the target regardless of its lateral movement. Since the application to eye surgery places the most stringent requirements on the tracker, the present invention is described mostly in reference to this application. However, it is to be understood that the invention is broadly applicable to any situation involving precision diagnostic measurements and/or laser operations on moving targets, including industrial applications, such as in semiconductor processing where laser annealing and other techniques call for precise alignment of a mask onto a substrate in the presence of vibrations.
In ophthalmic surgery, the ability to optically track or follow the movement of the patient""s tissuexe2x80x94not only the voluntary movements which can be damped with specialized treatment, but also the involuntary movements which are more difficult to control on a living specimenxe2x80x94is recognized as a highly desirable element in laser delivery systems designed to effect precision surgery in delicate ocular tissue. According to Adler""s Physiology of the Eye, even when the patient is holding xe2x80x9csteadyxe2x80x9d fixation on a visual target, eye movement still occurs. Such involuntary motion compromises the efficacy of certain ocular surgical procedures requiring great precision. This is true even with total immobilization of the eye, which is not fully effective in suppressing involuntary eye motion while being rather uncomfortable for the patient. Implementation of automatic tracking by remote means would therefore alleviate the need for such immobilization, while offering a method for more effectively accommodating all types of eye motion. Thus, augumenting surgery with on-line eye tracking option can improve significantly upon the accuracy and speed with which old surgical procedures could now be performed as well as enabling new procedures to be carried out for the first time.
In ophthalmology, it is also often desirable to image the tissue simultaneousely with positioning the treatment beam. Effective utilization of an imaging system capable of freezing on a display images or data relating to the configuration of the target during laser treatment requires that the target area be stabilized with respect to both imaging and the laser focal region, thus enhancing the accuracy of energy deposition in tandem with viewing sharpness. The ability to stabilize a video image of a moving target during the surgery procedure itself is especially desirable in those high precision laser interventions employing an instantaneous full image rather than a series of scanned images, such as described in co-pending U.S. patent application Ser. No. 843,374, which is incorporated herein by reference, and in U.S. Pat. No. 5,098,426.
In still other applications relating to diagnostics of targets, tracking can serve an important function in allowing cross-registration of successive readings taken across a moving target. By correlating the true positions of given target segments at the time the readings are taken, the effects of target motion can be compensated for via programming in the computer (i.e., software). In application to corneal surface mapping, utilization of transverse tracking, especially in concert with a depth tracking method that can keep the distance to the eye constant, e.g., as was disclosed in co-pending U.S. patent application Ser. No. 945,207, opens up the prospect of performing true point-by-point thickness and curvature measurements with standard scanning techniques. For example, by aligning separate readings relative to each other, accurate reconstruction of both anterior and posterior surfaces of ocular tissue such as the cornea or the lens can be feasible by scanning the eye with just a slit illuminator coupled to a CCD camera to detect each surface""s reflections. Using such simple instrumentation to perform simultaneous surface and thickness measurements was not possible prior to this invention.
Prior attempts to derive simultaneous pachymetry and topography information such as by D. J. Gormley et. al. in Cornea, vol. 7, pp. 30-35, 1988 using a scanning laser slit lamp and a photokeratoscope were clearly hindered, among other factors, by the lack of cross-referencing that only tracking can provide. Thus, to compensate for eye movement, each slit lamp image reading consisted of an average of a series of measurements, a procedure which could take up to several minutes. To map the entire cornea in this manner would then clearly require an inordinately long time, especially since, to maintain a common reference point between successive readings, the patient would have to stay fixated throughout the entire scan. By adding means for on-line tracking, the number of required readings can be significantly reduced to where a combined method of depth and surface profiling becomes practical, even allowing the possibility of utilizing a slit illuminator alone for both measurments.
In general, stabilization of a moving target requires defining the target, characterizing the motion of the target, and readjusting the aim of the optical system repeatedly in a closed-loop fashion. For ophthalmic surgery, requirements for a tracking system are set by the type of eye motion, which fall into three categories: microsaccades, drift and high frequency tremor. The high frequency tremors, of about 90 Hz set an upper limit to the frequency, but are of a very small amplitude (up to 40 seconds of arc). Microsaccades are highly accelerated motions with constantly changing directions but lower frequencies (a few Hz). These small but rapid eye movements, combined with slow drift (about 1 minute arc/sec), prevent the retinal image from fading. Analysis of measurements of peak velocity-magnitude-duration parameters by e.g., A. Terry Bahill et al. in Invest. Ohthalmol. Vis. Sci., Vol. 21, pp. 116-125, 1981, indicate that requirements set for lateral eye tracking should include, as a goal, the ability to respond to movement with accelerations of up to 40,000 deg/sec2. This translates to amplitudes of about 1 degree at maximum frequencies of 100 Hz and increasing to nearly 15 degree at 20 Hz. Meeting these response goals while maintaining accuracies on the order of 5-10 microns (as may well be desired in certain ocular surgical interventions near the visual zone), means that any moving parts within the tracker apparatus must not contribute internal vibrations, overshoots, or other sources of positioning error which could cumulate to an error in excess of the prescribed micropositioning tolerances. Other applications may impose tighter requirements on some parameters while relaxing others, depending on the margins of error set for a given process or procedure and the required overall system performance. We do expect, for example, that compensation over larger amplitudes and/or at higher frequencies may be desired in certain micromachining applications, which tighter requirements may, however, be offset by the ability to mark the workpiece to produce sharper tracking landmarks. Such target signal enhancements are not possible for ocular tissue without invading the organ itself, which is what makes the eye tracking application so uniquely demanding.
To date, no instrument has succeeded in tracking the human cornea in a cost effective manner. Previous attempts at achieving this result fall into one of two distinct categories, namely optical point trackers and correlation trackers, the latter including numerous variations of pattern recognition and edge detection methods. Optical point trackers utilize various lens-like properties of the eye to locate optically distinguished locations such as the first, second, third and fourth Purkinje points. For example, Crane and Steele (Applied Optics, Vol. 24, pp. 527, 1985) describe a dual Purkinje projection technique to compare the displacement of two different-order Purkinje projections over time, and a repositioning apparatus to adjust the isometric transformation corresponding to the motion. Similar application of dual Purkinje technique to a stabilized visual system was advanced by Crane (U.S. Pat. No. 4,443,075) using a fundus illumination and monitoring device. These and similar Purkinje image-based tracking methods purport to follow the movement of the anterior surface of the eye. While such techniques possess, in principle, sufficient speed to follow the displacement of Purkinje points, they do include an implicit assumption that the eye moves as a rigid body. In reality, however, the eye does not move as a rigid body, hence localization of the Purkinje points can be influenced by transient relative motions between the various optical elements of the eye, which leads to fictitious position information for identifying the surface of the cornea. In addition, such systems are rather complex and tend to exhibit large interperson variability in their calibration setting, which requires continuous real-time adjustments of the amplitude of the controlling signals.
The other class of tracking methods suggested in prior art involve, in one form or another, digital correlation techniques. These include retinal image trackers and various pattern recognition algorithms involving edge detection techniques. In either case, very fast frame-rate CCD cameras and sophisticated processing algorithms are required along with fast servo-controlled mirrors closing the loop. This is because, in general, methods based on pattern recognition are fundamentally digital, requiring high frequency updates. With the frequency response limited in practice to about one tenth the update frequency, digital signal comparisons are considered to be relatively slow. In the case of tracking eye motions, setting the sampling frequency to about an order of magnitude higher than the highest frequecy to be pursued translates into. kHz rates, leaving less than one thousandth of a second for processing the signal information.
Several other practical difficulties plague most pattern recognition techniques including the need for rather prominent and recognizable features, which are not easily located in any of the eye""s structures. Also, techniques predicated upon high speed correlation processing of video signals are often deficient due to unfavorable trade-offs between field of view and spatial resolution. Specifically, since the image processing algorithms are limited by the size and spacing of the view elements (pixels), the digital methods do not afford continuous resolution. Increasing the resolution exacts penalties in terms of the field of view. Yet, relatively large areas must be acquired or else a beam must be scanned. Consequently, the system is either light starved in the former case or else requires extremely regular and fast moving optical deflectors in the latter, along with complex electronic processing which can further limit the already slow response time of the system.
While there have been some claims of successful tracking for the retina (see for example, Milbocker and Feke in Applied Optics. Vol. 31, pp. 3719-3729, 1992, Katz et al. in American Journal of Ophthalmology, vol. 107, pp. 356-360, 1989 and A. Forster in Proc. of Ophthalm. Tech., SPIE vol. 1423, pp. 103-104, 1991), to our knowledge no instrument of this type has produced a real-time stabilized image of the cornea to date. Typically, CCD cameras are used to analyze the translations of an image on the retina from which the resulting coordinate transformation could be computed. The data is then fed to e.g., galvanometric driven mirrors which are repositioned so as to maintain a beam at the selected location of the subject. Other than the issues alluded to above regarding inherent limitations of digital edge detection techniques, the need in most of these instruments to utilize galvanometer drives to reposition mirrors at nominal kHz acquisition rates adds complexity to the system while further limiting the positioning accuracy due to overshoot problems.
Bille et al. (U.S. Pat. 4,848,340) describe a simpler variant on the pattern recognition method, whereby a grid is marked, using a laser, on the epithelial surface of the cornea, in a known reference alignment to the eye""s visual axis. An infrared optical system monitors reflections there-from, generating an error signal whenever the position of the mark deviates from reference alignment. The error signal is transmitted to a laser guidance system containing a fine tuner (consisting, e.g., of one or more galvanometrically controlled mirrors) which steers the laser beam in a manner that reduces the error signal to null. This type of a tracking system requires a sensor such as a photodiode array that can detect variations in the intensity of the reflected light pattern to generate signals representative of grid movements. These sensors suffer from both slow response times and limited spatial resolutions. Typically, with accuracy bounds set by the space between the array elements divided by the magnification, it is difficult to obtain resolutions better than 50 microns or so in practical systems. Furthermore, like any closed-loop feedback control that requires comparison of input signals to some reference, the technique taught by Bille et al. is digital in nature, which means that it suffers from similar drawbacks as image trackers and edge detectors in general, including processing speeds limited by the servo rate to less than a millisecond.
It should also be appreciated that a target tracking and laser positioning mechanism that relies on a mark on the surface of the cornea in order to perform corneal surgery, such as described by Bille""s tracking method, might lead to misdirected positioning of laser lesions below the surface when combined with any suitable focused laser. Thus, the mark would change its absolute location due to changes in the structure and shape of the material being modified that are caused by use of a laser surgery instrument itself, rather than by eye motions. It is therefore not clear that a tracking method based on marking the target tissue itself is compatible with laser surgical interventions performed simultaneousely with the tracking. Moreover, one of the features of the present invention is to enable non-incisive procedures inside target tissues by remote means. It would hence be counterproductive to mark the surface of the cornea for the sole purpose of following the motion of said mark.
In another embodiment of Bille et al. U.S. Pat. No. 4,848,340, the tracking is based on a reference provided by an empirically determined offset between the eye""s symmetry axis and the eye""s visual axis. It is claimed that tissue can be tracked by monitoring the reflection from the apex of the cornea, thus avoiding the need to mark the eye, and/or, rely solely on patient fixation. However, with this technique, the tracking does not follow tissue features generally corresponding to the targeted surgical site itself. Instead, Bille et al.""s technique tracks reference points that are, like Purkinje points, a property of the optical system and do not correspond to any particular physiological tissue. They are therefore separate, remote from and may be unrelated to the targeted surgical site. The accuracy of the tracking is thus compromised in direct proportion to the degree of the reference points"" remoteness relative to the surgical site, while ambiguities inherent in measurements of the symmetry and/or the visual axis will further reduce the accuracy with which positional changes of the targeted surgical site can be pursued using these methods.
By contrast with either pattern recognition based systems or optical point trackers, the methods of the present invention disclosed herein involve contrast tracking which does not rely on well-defined edges and/or patterns that must be compared to some reference. This allows great flexibility in selecting the tracking landmark, since prominent and constant edges are not required for acceptable signal-to-noise ratios. In the preferred embodiment, the tracking information is to be obtained through means contiguous to the target region, which is mechanically and structurally considered as part of it, but is unlikely to be affected by the course of the laser intervention. For example, the system and techniques disclosed herein resolve, for the first time, difficulties associated with previous attempts at limbus tracking. The limbus, located at the outer edge of the cornea, presents several advantages as a tracking landmark for corneal procedures. It is actual tissue that is contiguous to the targeted corneal tissue and is expected to provide a faithful representation of non-surgically induced displacements. Yet it is located far enough from the site of operations so that the transient displacements occasioned by the impact of the laser pulse on the target site will be damped sufficiently to avoid inducing fictitious tracking signals. Prior limbus trackers have not been successful because the limbus is a poor candidate for any technique relying on edge detection, constituting not a sharp boundary but a transition zone between the cornea and the sclera. Therefore, poorly defined edges and shapes that appear to deform due to rotations have led to difficulties in extracting signals out of noise.
The present invention overcomes these shortcomings because it is practical even in the absence of prominent, well-defined or even temporally constant edges. The only two requirements are that sufficient contrast be present, and that the feature possess a degree of symmetry. In the eye, these conditions are fulfilled by e.g., the limbus structure and in most cases, the pupil as well. In our co-pending patent application Ser. No. 843,374, a method for tracking the limbus was disclosed relying on a set of two quadrant detectors as the position sensor. The present patent application expounds on that disclosure by highlighting a unique electronic control system that can be used for the tracking to great advantage, and including as a desirable feature a dual feedback loop that can be all analog, thus significantly increasing the practical speed of operations over comparable digital methods. Along with the added emphasis on the electronic means for realizing rapid limbus tracking, the simplified signal processing and fast logic operations involved in the electronic servo loops of the present invention also allow a substantial expansion of the scope of the previously disclosed limbus tracking method to include other contrast-based tracking landmarks and alternative position sensing detectors as may be needed to implement tracking in different surgical, diagnostic or industrial settings.
Thus, it is an object of the present invention to provide means for stabilizing the lateral motion of randomly moving targets, allowing an imaging system, diagnostics illumination, and/or laser beam to maintain a lock on the target area, regardless of its movement.
It is further an object of the present invention to provide a means for recording eye movements in real time and in which data can be stored and manipulated for the purpose of compensating for lateral target motion by either hardware or software means.
It is a more specific objective to provide within the tracking system a means for sensing contrast in recognizable large scale boundaries such as the change between the cornea/sclera interface (limbus), thereby to determine the absolute location and orientation of these boundaries, all without having to resort to digital sampling techniques.
It is yet another object to identify the target""s velocity vector with sufficient speed and accuracy to be able to guide positioning optics, such as a pivoting mirror, to pursue and compensate target motions in a plane. (also referred to herein as the X-Y plane) perpendicular to the optical (Z) axis of the system as defined, most often, by a final objective lens assembly. The moveable optical element is under the directional control of a rapid servo device.
Still another specific object of the invention is to enable, in a preferred embodiment, all analog execution of electronic servo functions, if desired, including those that provide directional control of the moving optical element, thus circumventing difficulties associated with the slower all-digital closed loop servo systems.
Consistent with these objects, an embodiment of the present invention is herein disclosed, comprising a method and apparatus for precise lateral target tracking useful to ocular surgery and ophthalmic diagnostics, as well as to several other medical and industrial procedures. The techniques disclosed are particularly powerful in combination with an axial (Z) tracking system, such as the modified confocal microscopy-based technique disclosed by Wm. D. Fountain in U.S. Pat. No. 5,162,642 and in U.S. patent application Ser. No. 945,207. The Z-tracker discussed therein is fully compatible with the present system with which it can interface by way of suitable coupling optics and/or the tracking optical element itself, provided that the objective lens assembly is inserted as the first element in front of the target. The present invention also contains provisions, such as additional coupling optics, for interfacing the X-Y tracker with any number of other optical subsystems including, but not limited to, imaging means, laser targeting and treatment means and topography means. Such interfaces and the computerized methods required to implement them in practice, were described in co-pending U.S. patent application Ser. No. 843,374, which is incorporated herein by reference.
In application to laser surgery, tracking by following the subject eye tissue, i.e., recognizing new locations of the same tissue and readjusting a movable element that is optically connected to a surgical laser, assures that the laser will aim at the new correct target location and not deviate from a pre-programmed pattern an unacceptable distance. By also taking advantage of the coupling optics provided within the present apparatus to interface with a viewing assembly, such as a zoom microscope, the tracking system of this invention fulfills an important dual-use purpose: it can greatly enhance the precision of lesion placement in targets as well as improve safety margins by allowing simultaneous presentation of a stationary view (e.g., in the form of continuousely updated video image) of a given section of tissue on a display monitor. It is anticipated that the availability of fast and reliable tissue diagnostics simultaneousely with the surgery itself will be especially valuable in the specific case of ocular surgery (and concerning, in particular, corneal procedures), where obviating the need for eye immobilization, and all the drawbacks thereof, is a much desired feature.
In application to topographic and other diagnostic measurements, the transverse tracker of this invention can be operated in a manner allowing determination of target movement relative to a given coordinate system which is in a known reference to, e.g., the optical axis of the apparatus. Such movement can then be compensated for by software means using programming in the computer. The ability to actually compute and store information concerning target movement in a meaningful way is a valuable tool whenever sequential series of measurements are to be performed by way of e.g., scanning the target while it is moving, so that the results can be extrapolated to reveal a global feature of said target. Such an approach can be profitably applied to topographic and pachymetric measurements aimed at reconstructing, on a point-by-point basis, the curvature, shape and/or thickness of intra-ocular structures such as the cornea and the lens. Providing enhanced cross-registration of adjacent target segments, each referenced to the actual location of the target area at the time the reading is taken, promises to extend surface mapping accuracies to well beyond any existing corneal modeling systems. Furthermore, by interfacing the tracker with other existing instrumentation such as slit lamps and cameras, a host of new possibilities opens up for cross-correlating various optical target properties across wider areas and with higher local accuracies than was demonstrated with any prior art methods.
In application to industrial operations, the system and methods of the present invention can be used successfully to suppress vibrations in various semiconductor processing applications involving micron and submicron precision levels. Implementation of a tracker to detect and compensate for random enviromental vibrations may alleviate the need for expensive and cumbersome isolation equipment, thus leading to significant reductions in capital outlay costs while improving overall yields. These are desirable objectives in operations ranging from automated inspection and short repair of microchips to photolithography and laser annealing.
Like most servo-based tracking systems, the system and method of the present invention rely on returning an error signal generated by movement of the target to zero. However, this is where similarities to other tracking methods end. The key novel feature embodied within the present invention which distinguishes it from prior art concerns an approach to generating tracking signals by way of detecting variations in target contrast, thereby avoiding the need for complex edge detection algorithms. Consequently, position sensing detectors with continuous resolution elements can be utilized, thus eliminating information gaps while avoiding the need for high update frequencies. Also, the electronic control system is compatible with fast, simple, all-analog techniques for processing the signals from position sensing detectors and/or transducers and translating them into command signals for a servo control mechanism. Such an analog option is not feasible with other tracking methods based on digital data acquisition systems.
The tracking system is at least comprised of an illumination source, a sensor, a moveable optical element, such as a mirror, a two-dimensional logic board, and a dedicated microprocessor or logic board including appropriate signal processing firmware and software. Interfaces with other system elements such as laser aiming, target viewing and/or depth ranging subsystems may all be included as part of the optical train, while suitable control ports provide electronic interface with these other functions as well as with a central computer (using A/D converters where required). The transverse tracker disclosed herein is thus best considered as a module ready to be integrated with other system functions, be they diagnostic or therapeutic in nature.
In a preferred embodiment of the invention, the illumination light is projected off-axis relative to the optical axis of the instrument so as to minimize interference from either the treatment light (typically an intense laser beam) or depth ranging light (which can also be a laser, such as a low power HeNe). This configuration leaves considerable flexibility as to the choice of the tracking light source and may include lamps, LED""s and laser diodes, depending on the reflectivity properties of the tracking landmark used and the overall application needs. The position sensors are also selectable from among a class of position-sensiing photodetectors, all possessing fast response times (of well below a tenth of a millisecond) and excellent resolutions (on the order of 1 micron), with a view towards optimizing a given system performance. In one preferred embodiment of the invention, the transverse X-Y tracking detector consists of a set of two high speed quadrant detectors which provide an ideal match to tracking the limbus of the eye. Alternative detectors, including lateral-effect and superlinear position sensors, may however be selected for eye tracking or other applications, without compromising the effectiveness of the present invention.
An X-Y logic board comprises a key element of the electronic processing serving as the central xe2x80x9cswitchboardxe2x80x9d of the servo tracking loop. This is where voltage signals from the detectors corresponding to target motion are received and converted to appropriate commands for controlling the tracking optical element assembly drivers so as to compensate for said target motion by repositioning an movable optical element, such as a mirror. Since analog control signals can be used, processing speeds are not a limiting factor, even for applications requiring high repetition rate ( greater than 1 kHz) lasers. Thus, laser firing can always be disabled, if necessary.
In alternative embodiments, either piezoelectric, electromagnetic or galvanometric drives can be used to steer the moveable system element, such as a mirror to drive the error signal generated by target motion back to null. This component must however be capable of moving with sufficiently high acceleration and velocity to compensate for the fastest motion possible by the intended target. Since the operation of the system is not limited by either the processing speed of the servo loops or the detectors"" response time, the driven optical element with its finite moment of inertia is likely to provide the main limitation on the extent and rate of motion which can be compensated, so considerable care must be exercised in selecting the type of drive to match a given application needs. Nonetheless, the manner of operation and the principles of the present invention do not depend on specific drive mechanis, a particular transducer, or the moveable optical element, all of which components can therefore be selected to match the needs of specific applications.
Intrinsic to the tracking scheme disclosed herein is the choice of a suitable tracking landmark possessing of sufficient contrast under illumination by the light source used. This is a relatively easy condition to fulfill, lending the system considerable flexibility with regard to said choice. Ideally, the tracking landmark would be located contiguous and in proximity to the targeted tissue, yet without being coincident with the precise target site itself, since this site will change during the course of the intervention. For one preferred embodiment involving corneal procedures such as refractive surgery, the eye limbus at the rim of the cornea provides sufficient contrast to serve as the tracking landmark for procedures affecting either the central or peripheral portions of the cornea. Alternative landmarks such as the pupil, which may be useful when procedures in zones peripheral to the visual axis are contemplated, also fall under the domain of this invention. For still other, non-surgical interventions, a suitable feature can usually be artificially impressed upon the workpiece in sufficient proximity to the target site and with enough contrast to allow the efficatious employment of the tracking methods discussed in this invention.
It has been determined that target motions with accelerations of up to 40,000 deg/sec2, corresponding to rates of over 100 Hz for amplitudes of nearly 1 mm along two axes (or larger amplitudes but at lower rates), at accuracies of 5-10 microns can be pursued with the described embodiments of the invention, which should more than adequately compensate for all types of involuntary eye motion. Such speed and precision were not attainable in a predictable, systematic manner with any of the prior instruments or practices used.